1. Field of the Invention
The present invention relates to an alkaline storage battery, and particularly to an improvement of positive electrode used for the battery.
2. Description of Related Art
With the recent spread of portable devices, alkaline storage batteries are demanded to have higher capacity. Particularly, nickel metal hydride storage batteries are secondary batteries which comprise positive electrodes mainly composed of nickel hydroxide and negative electrodes mainly composed of a hydrogen-absorbing alloy, and they have spread as secondary batteries of high capacity and high reliability.
The positive electrodes for the alkaline storage batteries will be explained below.
The positive electrodes for alkaline storage batteries are roughly classified into two types of sintered type and unsintered type. The former are prepared by sintering a core material such as a punching metal and a nickel powder to obtain a nickel sintered substrate having a porosity of about 80%, impregnating the resulting substrate with an aqueous solution of a nickel salt such as aqueous nickel nitrate solution and then dipping the substrate in an aqueous alkali solution, thereby to produce nickel hydroxide in the porous nickel sintered substrate. Since the porosity of the substrate in the positive electrodes cannot be increased to more than the above porosity, amount of the nickel hydroxide cannot be increased and there is the limit in increase of capacity.
The latter unsintered type positive electrodes are prepared by holding nickel hydroxide particles as an active material in a three-dimensionally continuing foamed porous substrate having a porosity of not less than 95% as disclosed in JP-A-50-36935, and, at present, they are widely used as positive electrodes of secondary batteries of high capacity. In these unsintered type positive electrodes, the porous substrate is filled with spherical nickel hydroxide from the point of attainment of high capacity.
Moreover, it is generally conducted to use a metal ion such as cobalt, cadmium, zinc or the like which is dissolved in the nickel hydroxide particles to partially form a solid solution for the improvement of discharge characteristics, receptivity for charging and life characteristics.
The pore size of the foamed porous substrate is about 200-500 xcexcm, and the pores are filled with spherical nickel hydroxide having a particle diameter of several xcexcm to several ten xcexcm. In nickel hydroxide in the vicinity of the nickel metal skeleton, collection of electricity is maintained and the charge and discharge reaction smoothly proceeds. However, the reaction of nickel hydroxide apart from the skeleton does not proceed sufficiently. Therefore, in order to improve utilization ratio of the nickel hydroxide, a conductive agent is used to perform satisfactory electrical connection of the nickel hydroxide particles which are apart from the skeleton.
Cobalt compounds such as cobalt hydroxide and cobalt monoxide, metallic cobalt, metallic nickel and the like are used as the conductive agent. Thus, it becomes possible to fill the active material at a high density in unsintered type positive electrodes and improve utilization ratio, and capacity can be increased as compared with the sintered type positive electrodes.
Furthermore, JP-A-8-148145 discloses a method for producing an active material of a positive electrode for high capacity nickel-hydrogen storage batteries which are excellent in overdischarge characteristics and meet the market demand for improvement of cycle characteristics, which comprises coating a cobalt compound on an active material nickel hydroxide and subjecting the cobalt compound to an alkali oxidation treatment to convert the compound to a higher order cobalt compound, and JP-A-9-73900 discloses an improvement of the above method.
According to these methods, the nickel hydroxide powder coated with the cobalt compound is sprayed with an aqueous alkali solution under fluidization or dispersion in the heated air. As a result, it has become possible to make alkaline storage batteries of high energy density which are improved in utilization ratio of active material and battery characteristics such as high rate discharge characteristics as compared with the conventional methods in which the cobalt compound is added as an external additive.
Moreover, in alkaline storage batteries, a phenomenon of reduction of charging efficiency occurs when temperature of the batteries is high. For solving this problem, the electrolyte used in nickel-hydrogen storage batteries is optimized or calcium compounds or rare earth oxides such as yttrium oxide and ytterbium oxide which improve high-temperature charging efficiency are added to positive electrode active materials. This is disclosed, for example, in JP-A-9-92279.
The above construction can provide high capacity and can improve charging efficiency at high temperatures, but there has still been a problem of reduction of discharge characteristics.
The main object of the present invention is to provide an alkaline storage battery improved in discharge characteristics with maintaining the discharging efficiency at high temperatures. These and other objects of the present invention will be readily apparent from the description of the invention.
For attaining the above object, the present invention provides an alkaline storage battery which uses a positive electrode comprising an active material containing nickel hydroxide particles and a rare earth element or a compound thereof containing a trace amount of Fe or a Fe compound.
According to the above construction, it becomes possible to provide an alkaline storage battery improved in discharge characteristics with maintaining the charging efficiency at high temperatures.